1. Field of the Invention
The present invention relates to a nickel powder, in particular, nickel powder suitable for forming electrodes in multilayer ceramic electronic components, such as multilayer capacitors, multilayer inductors, and multilayer actuators, to a conductor paste comprising the nickel powder, and a multilayer ceramic electronic component using the same.
2. Description of the Related Art
A multilayer ceramic electronic component (also referred to hereinbelow as “multilayer electronic component”) is usually manufactured in the manner as follows. A ceramic raw material powder such as dielectric, magnetic, or piezoelectric material powder is dispersed in a resin binder and formed into sheets to prepare ceramic green sheets (referred to hereinbelow as “ceramic sheets”). A conductor paste for an internal electrode that is prepared by dispersing an inorganic powder comprising an electrically conductive powder as the main component and, optionally, a ceramic powder or the like in a vehicle comprising a resin binder and a solvent is printed according to a predetermined pattern on the ceramic sheets and dried to remove the solvent and form a dry film of the inner electrode. A plurality of ceramic sheets, each having the dry film for the inner electrode that were thus obtained, are laminated and pressurized to obtain a non-fired laminate in which the ceramic sheets and paste layers of inner electrodes are alternately laminated. The laminate is cut to a predetermined shape, then subjected to a binder removal process in which the binder is burned and dissipated, and fired at a high temperature whereby sintering of the ceramic layers and formation of the inner electrode layers are conducted simultaneously and a ceramic body is obtained. Terminal electrodes are then fused to both end surfaces of the body and a multilayer electronic component is obtained. The terminal electrodes and the unfired multilayer body are sometimes simultaneously fired (i.e., co-fired).
In recent years, powders of base metals such as nickel and copper are mainly used instead of powders of noble metals such as palladium and silver as electrically conductive powders of conductor pastes for inner electrodes. Accordingly, firing of the laminate is usually carried out in a non-oxidizing atmosphere with an extremely low partial pressure of oxygen in order to prevent the oxidation of the base metal during firing.
As a demand for miniaturized, highly multilayer electronic components has been growing in recent years, a rapid transition has been made to reducing the thickness of layers in both the ceramic layers and the inner electrode layers, in particular, in multilayer ceramic capacitors using nickel as an electrically conductive powder. As a result, ceramic sheets of smaller thickness came into use and also extremely fine nickel powder with a particle size of 1 μm or less, and even 0.5 μm or less, came into use for conductor pastes for inner electrodes.
Nevertheless, with such an extremely fine nickel powder, the nickel particles are over-sintered and lead to particle growth during the firing of a capacitor, which creates large voids in an internal electrode, and another problem is that the electrode ends up being thicker, so there is a limit to how much the thickness can be reduced.
Also, a fine nickel powder is highly active, and its sintering commencement temperature is extremely low. Particularly when it is fired in a non-oxidizing atmosphere, sintering and shrinkage commence at an early stage (such as at a low temperature of 400° C. or below), even with single crystal particles of relatively low activity. On the other hand, the ceramic particles that make up ceramic sheets generally start to undergo sintering at a much higher temperature than nickel powder particles. Therefore, when ceramic sheets and an internal electrode paste containing the above-mentioned nickel powder are co-fired, since the ceramic layer does not shrink together with the nickel film, the nickel film is pulled in the planar direction. Consequently, it is believed that voids produced in the nickel film by sintering at a relatively low temperature tend to expand into large voids as sintering proceeds in a higher temperature range. When such large voids are produced in an internal electrode, the electrode becomes discontinuous, which raises its resistance or leads to disconnection, so the electrostatic capacity of a capacitor decreases.
Furthermore, because of volumetric expansion and shrinkage caused by the redox reaction of nickel during firing, the sintering shrinkage behavior of the nickel film does not match that of the ceramic layer, and this results in delamination, cracking, and other such structural defects, and decreases the yield and reliability.
To solve these problems, Japanese Publication No. 2000-45001 A, for example, discloses that if a dense oxidation film having a certain thickness is formed on the surface of a nickel powder, changes in the volume and weight due to redox of the nickel during firing will be kept small, and the sintering commencement temperature will be raised, allowing delamination to be effectively prevented.
Also, Japanese Patent Publication Nos. 11-80816 A, 11-80817 A and 2006-37195 A disclose nickel powders containing sulfur. For example, Japanese Patent Publication No. 2006-37195 A discloses that a compound layer containing nickel and sulfur, such as Ni—S or Ni—S—O, is provided on the surface of a nickel powder by treating the powder with a sulfur gas or a sulfur compound-containing gas, preferably after surface oxidation. It is stated that this surface layer containing nickel and sulfur suppresses oxidation and reduction of the nickel powder during firing, and particularly in the binder removal step, and raises the sintering commencement temperature, so the oxidation behavior, reduction behavior, and sintering behavior of the nickel powder are improved and, as a result, delamination is suppressed in the course of manufacturing a multilayer ceramic capacitor.
The oxidation layer formed on the nickel powder surface as mentioned in Japanese Patent Publication No. 2000-45001 A lowers the activity at the nickel surface, and therefore reduces structural defects in multilayer electronic components, and is effective at preventing an increase in resistance of an internal electrode, but the effect is still not satisfactory. In particular, if the nickel particle size is on the submicron order, and especially if it is 0.5 μm or smaller, then it becomes impossible to suppress the disconnection of the electrodes and, furthermore, there may be problems such as a decrease in reliability, the generation of structural defects, and the deterioration of capacitor characteristics, which are thought to be due to the incomplete decomposition of the vehicle components in the binder removal step. Specifically, even though it has an oxidation layer, an extremely fine nickel powder such as this is highly active, so it acts as a decomposition catalyst on the vehicle, and the resin may explosively decompose at temperatures below its ordinary decomposition temperature. If this happens, the sudden generation of gas can cause cracking or delamination. Also, because the reaction is so sudden, the resin may not completely volatilize in the binder removal step, leading to carbonaceous residues such as carbon or carbon compounds, and this pulls out the oxygen from the ceramic layer in the volatilization by oxidation and gasification during the subsequent step of sintering the ceramic at a high temperature, which can lower the strength of the ceramic material or adversely affect the electrical characteristics such as electrostatic capacity and insulation resistance. Also, the residual carbon lowers the melting point of the nickel powder and may cause over-sintering.
A nickel powder obtained by the method disclosed in Japanese Patent Publication No. 2006-37195 A has on its surface a compound containing nickel and sulfur, such as Ni—S or Ni—S—O, the effect of which is that redox of nickel is suppressed and sintering is delayed, in addition to which, research by the inventors of the present invention has revealed that the above-mentioned sudden combustion of the resin at low temperature during binder removal can be suppressed. However, a problem with a conductor paste in which a nickel powder such as this is used is that the nickel undergoes oxidation when the binder removal is performed in an atmosphere that includes oxygen. Specifically, for the binder to be removed efficiently, the binder removal is usually carried out in an oxidizing atmosphere, such as air or nitrogen containing several percent oxygen. The above-mentioned nickel powder, however, does not have sufficient oxidation resistance, and may undergo oxidation in these atmospheres. If the nickel is excessively oxidized in the binder removal step, then when it is subsequently fired at a high temperature in a reducing atmosphere, reduction of the oxides will generate a gas and change the volume, so that a dense electrode is not obtained, and will also cause cracking and delamination in the multilayer electronic component.